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Deepwater (Turbidite) Sandstone Elements of the Jackfork Group in Arkansas: Application to Exploration and Development in Eastern Oklahoma
Jackfork Group strata in eastern Oklahoma are now recognized as important natural gas reservoirs. Gas production can be related to large scale structures, as well as smaller scale fractures. There is also the potential for stratigraphic trapping of gas, and for a sedimentary control on fracture frequency.
Excellent exposures of Jackfork Group strata in southwest-central Arkansas provide insights into analog reservoir strata in eastern Oklahoma. The three major deepwater (turbidite) elements that are found in most oil and gas reservoirs also occur in the Jackfork Group: channel-fill strata, sheet sandstones, and levee/overbank deposits. Examples of each are presented in this paper, along with sedimentologic and stratigraphic criteria by which to distinguish them.
Outcrop gamma ray logs obtained with a standard logging truck and with a hand-held gamma ray scintillometer of the Jackfork elements reveal a lack of distinguishing features. This is a common trait of conventional well logs because the distinguishing features are often beneath the resolution of the logging tools. Cores, and less expensive borehole image logs, provide the necessary resolution to distinguish these features. To demonstrate this principle, examples are presented of borehole image logs of deepwater deposits from the Pliocene of the Gulf of Mexico and from the Cretaceous Lewis Shale of Wyoming.
Identifying the turbidite elements from borehole data or cores is critical to improving volumetric calculations because each element will differ in areal and stratigraphic extent. Equally important, different development drilling scenarios are required for each element to successfully drill into target sandstones and to maximize production. Also, there is some evidence that fracture frequency varies within different elements. Thus, the use of borehole image logs, or cores, is justified for improved economic evaluation and efficient development of Jackfork Group reservoirs.
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